This invention relates to a recording system for a multichannel record disc and more particularly to a system for recording a discrete multichannel record disc with a compensation for tracing distortion which is recorded to prevent occurrence of tracing distortion of the reproducing stylus.
The applicant had previously proposed a recording and/or reproducing system for four channel record discs as disclosed in U.S. Pat. No. 3,686,471, issued Aug. 22, 1972, and entitled "System for Recording and/or Reproducing Four Channel Signals on a Record Disc." This system is already being used on a worldwide basis.
By this system, two channels of sum and difference signals are formed respectively from the four channels. More specifically, the four signals of the first through fourth channels are respectively denoted by the notations CH1, CH2, CH3, and CH4. Sum signals (CH1 + CH2) and (CH3 + CH4) and difference signals (CH1 - CH2) and (CH3 - CH4) are formed from these four signals.
Thereafter, the difference signals are angle modulated, to obtain angle modulated wave difference signals F(CH1 - CH2) and F(CH3 - CH4) of a band higher than the above mentioned direct wave sum signals. These signals are mixed with the direct wave sum signals (CH1 + CH2) and (CH3 + CH4). Two sets of multiplexed signals {(CH1 + CH2) + F(CH1 - CH2} and {(CH3 + CH4) + F(CH3 - CH4)}of these direct wave sum signals and angle modulated wave difference signals are recorded by respectively cutting them on the left and right walls of a single groove on a record disc of the 45-45 type.
The direct wave sum signals have a frequency band from 0 to 15 KHz, while the angle modulated wave difference signals have a frequency band from 20 to 45 KHz.
In general, as is known, when a reproducing stylus traces the sound groove of a record disc during reproduction, a tracing distortion inevitably occurs. This distortion occurs since the tip of the reproducing stylus is not an ideal point but has, of course, a finite radius of curvature. The locus of the center of the stylus does not coincide with the shape of the sound groove. Harmonics are produced as a natural result of the occurrence of this tracing distortion, such as the second, third, harmonics etc.
When a tracing distortion is produced during reproduction, harmonic components of the direct wave signal tracing distortion enter into the band of the angle modulated wave signal. Furthermore, distortion components are produced due to mutual interference between the direct wave signal and the carrier wave signal. The harmonics of the angle modulated wave signal can be removed by a band-pass filter which passes only the angle modulated wave signal. The harmonic components of the direct wave signal enter the angle modulated wave signal band and cannot be removed by the filter. When the tracing distortion harmonic become admixed in the angle modulated wave signal band in this manner, the distribution of the side-band energy of the angle modulated wave signal changes. Responsive thereto, the tone quality of the reproduced demodulated signal deteriorates.
Accordingly, the applicant had previously proposed a system for applying a reverse distortion deriving the recording four channel signals on a record of disc. Therefore, the tracing distortion which occurs at the time of reproducing, the distortions originally recorded cancel each other. A reproduced signal is obtained which is exactly the same as the intended recording signal. In this previously proposed system, a tracing distortion compensation (tracing distortion neutralization) is provided at a stage after a mixer for mixing and multiplexing the above mentioned direct wave sum signals and the angle modulated wave signal. The tracing distortion compensation circuit applies an artificial compensation for tracing distortion to each of the direct wave sum signals to the angle modulated wave signal.
The above mentioned distortion compensation circuit unavoidably imparts a delay to the signal because it employs delay circuits. By this proposed system, however, the distortion compensation must be carried out over the entire band of the direct wave signal band and the angle modulated wave signal band. For this reason, the distortion compensation circuit must compensate over the entire frequency band. At the same time, the delay time must be constant over the entire frequency band.
However, it is difficult to realize a distortion compensation circuit having a uniform delay time over the entire band of from 0 to 45 KHz. For example, one problem heretofore encountered has been that the delay time for the angle modulated wave band becomes different from the delay time for the direct wave band. Thus, a deleterious effect is imparted to the angle modulated wave band, and thorough distortion compensation cannot be carried out.
Furthermore, a signal which has undergone tracing distortion compensation assumes a waveform having sharp projections at the crests (or valleys) even when the original signal is of, for example, a sinusoidal waveform. Accordingly, even when tracing distortion compensation is carried out electrically on an angle modulated wave signal of high frequency of from 20 to 45 KHz as described above, there is the problem of the cutting mechanism. The cutting is not capable, itself, of faithfully and completely tracing a signal of a waveform as described above. Thus, when tracing distortion compensation is carried out for angle modulated wave signals, there is almost no effect.
In accordance with the present invention, the above mentioned tracing distortion compensation is carried out with respect to only the direct wave signal. As a result, the distortion compensation characteristic and the delay characteristic need be matched only to a band of 0 to 15 KHz. For this reason, the organization of the distortion compensation circuit is facilitated.
On the other hand, if a sound groove of a waveform, wherein a direct wave signal and an angle modulated wave signal are superimposed as described above, is reproduced by a reproducing stylus having a certain tip radius, the direct wave signal imparts a phase modulation to the angle modulation signal as described hereinafter.
In accordance with the present invention, for the purpose of mutual cancellation at the time of reproducing of the above mentioned phase modulation, recording is carried out with the addition beforehand of a signal for this mutual cancellation.